The Manual on Uniform Traffic Control Devices (MUTCD) is a document issued by the Federal Highway Administration (FHWA) of the United States Department of Transportation (USDOT) to specify the standards by which traffic signs, road surface markings, and signals are designed, installed, and used. These specifications include the shapes, colors, and fonts used in road markings and signs. In the United States, all traffic control devices must generally conform to these standards. The manual is used by state and local agencies as well as private construction firms to ensure that the traffic control devices they use conform to the national standard. While some state agencies have developed their own sets of standards, including their own MUTCDs, these must substantially conform to the federal MUTCD.
The Congress of the United States has acknowledged the importance of nighttime visibility of road signs and markings as important safety tools. The 1993 DOT Appropriations Act stated that the “Secretary of Transportation shall revise the MUTCD to include a standard for a minimum level of retroreflectivity that must be maintained for traffic signs and pavement markings which apply to all roads open to public travel.”
Table I shows the present federal MUTCD standard for pavement marking retroreflectivity:
TABLE IMinimum Maintained Retroreflectivity Levels1 forLongitudinal Pavement Markings (2010)Posted Speed(mph)≦3035-50≧55Two-lane roads with centerline markingsn/a100250onlyAll other roadsn/a50100Measured at standard 30-m geometry in units of mcd/m2/lux, where mcd is the intensity of the light source, m2 is the square of the distance in meters between the light source and the reflector, and lux is the illumination falling on a unit area.
It is known in the art of pavement marking to apply a retroreflective marking by a method comprising the steps of a) spraying the area with a hot epoxy binder, b) depositing a particulate ceramic retroreflective element onto the fresh epoxy binder, and c) depositing spherical glass beads of a specified sieve size distribution onto the fresh epoxy binder. The beads and ceramic elements are intended to function as mirrors, reflecting light from vehicle headlights back to the light source vehicle.
It is further known to apply such a marking to an area of pavement that has been ground to a level significantly below the normal pavement surface, to protect the marking from traffic wear and abuse by snowplow blades. The pavement typically may be asphalt, concrete, or other material used as a highway wearing surface, and the marking may be, for example, a roadway center stripe, edge stripe, intermittent lane divider, arrow, alphanumeric indicia, or any other pavement marking permitted by the Federal MUTCD.
A new application of such a prior art marking, under dry weather conditions, can yield a measured retroreflectivity of about 320. The average value over three years of use can be about 160, with a minimum value of about 50 mcd.
A significant shortcoming of such a prior art marking is that there may be almost no retroreflectance at night when the pavement is wet.
An additional complication is that a driver's need for light is known to approximately double every 13 years, e.g., at age 85 a driver needs 8 times as much light as at age 46. Further, 12% of the country's drivers are over the age of 65.
What is needed in the art is an improved system for pavement marking wherein the retroreflectivity is increased over that provided by prior art marking systems and also under all weather, illumination, and pavement conditions.